[codex] update default openai model

[timrogers]

What changed

• Updated the default OpenAI model from gpt-4o to gpt-5.4-mini in src/llm.rs.

Why

• This keeps the OpenAI-backed inference path aligned with the newer default model requested for the project.

Impact

• Users who run Formanator with an OpenAI API key will now use gpt-5.4-mini by default for claim inference.
• No CLI flags or configuration values changed.

Validation

• cargo test --locked --all-features
• cargo build --release --locked
• pre-commit run --all-files could not run in this environment because pre-commit is not installed.